This invention relates to microwave pulse generators and to pulse-echo ranging systems, such as are used for determining levels of material interfaces in tanks, silos, channels and bodies of liquid, using such generators.
One well known technique for such ranging utilizes pulses of acoustic energy which are echoed from the interfaces to be detected. The resolution of any such system is limited by the wavelength of the radiation utilized, and acoustic systems have the advantage that, particularly at ultrasonic frequencies, wavelengths are short, providing good resolution, while the speed of sound is comparatively low, facilitating processing of the echo signal in real time. On the other hand, the speed of sound is affected substantially both by temperature and the nature of the medium through which it passes, and the reflection and absorption characteristics of materials and their interfaces may not be satisfactory. For example, such systems often have difficulty in reliably detecting the surface of liquid covered with foam.
An alternative approach uses radar type techniques using pulses of microwave electromagnetic energy. The speed of propagation of such energy (the speed of light) is little affected by the temperature or composition of the propagation medium, and reflection characteristics are sufficiently different to enable some of the problems encountered with acoustic systems to be overcome. On the other hand, microwave wavelengths are such that extra high frequencies are needed to provide adequate resolution, and the very short echo times involved require quite different processing techniques. Furthermore, available microwave generators tend either to produce inadequate power, or to be unduly expensive and/or produce excessive power for typical short-range ranging applications, which require far less power than typical radar systems, but more than can be provided by semiconductor microwave oscillator components that are available at economically acceptable prices.
It is an object of the present invention to provide an economical microwave pulse generator capable of providing adequate power for industrial ranging applications, together with a ranging system utilizing such a generator which can utilize known ultrasonic receivers and signal processing techniques such that basically similar signa processing techniques, receivers and transducer wiring may be used for both microwave and acoustic operation, the acoustic transducers used in acoustic systems being replaced by more or less functionally equivalent microwave heads.
According to the invention, a microwave pulse generator comprises a reverse biased semiconductor P-N junction coupled into a microwave resonator, and a trigger circuit effective to initiate a non-destructive avalanche discharge through said junction. Further features of the invention will be apparent from the following description and the appended claims.